In mechanical seals, where, for example a first sealing ring is mounted in fixed axial and rotational relationship on a shaft and a second sealing ring is mounted in fixed rotational relationship to a housing, the second sealing ring being movable axially of the housing, so that it may be urged axially into sealing engagement with the first sealing ring, there is a requirement to provide a secondary seal between the second sealing ring and the housing, which will permit axial sliding of the second sealing ring.
Hitherto, spring energised plastic seals have been used for this purpose. Such seals comprise a plastic ring defining a pair of sealing elements, the sealing elements being urged apart by spring means. The spring energised plastic seal is located in an annular groove in the housing, one sealing element being urged into sealing engagement with the base of the groove and the other sealing element being urged into sealing engagement with the second sealing ring. The plastic ring is disposed so that the pressure of fluid sealed, will reinforce the load applied on the sealing elements by the spring means.
When spring energised plastic seals of this type are subject to high temperatures and pressures, the plastic ring readily deforms to the shape of the groove, taking on a permanent set. When the system cools, the plastic ring, which has a high coefficient of linear expansion, can shrink a considerable amount and grip the second sealing ring, restricting axial movement of the second sealing ring.
In hydrodynamic seals or gas seals which rely on a cushion of fluid being built up between the faces of the sealing rings, this presents a serious problem when starting up from cold, the spring energised plastic seal preventing separation of the faces, so that they will be subjected to significant wear, which may result in premature failure of the seal.
While the spring energised plastic seal according to the present invention is particularly advantageous with hydrodynamic or gas seals, it is applicable to any application where a spring energised plastic seal may be used, particularly where the spring energised plastic seal is liable to be exposed to high temperatures and pressures.